1. Field of the Invention
The present invention relates to a process for producing motor fuels and motor fuel components from hydrocarbons which contain tertiary olefins in a mixture with saturated and olefinically, diolefinically and acetylenically unsaturated hydrocarbons, the tertiary olefins being etherified with one or more C.sub.1 -C.sub.4 -alkanols and the unsaturated constituents being largely hydrogenated.
2. Description of the Related Art
It has been known for a long time to react tertiary olefins with lower alkanols to give alkyl tert.-alkyl ethers. It is also known that, in the case of methyl tert.-butylether (MTBE), the equilibrium lies largely on the side of the ether at not unduly high temperatures, whereas even in the case of tert.-amyl methyl ether (TAME) at the same temperature, substantially smaller quantities of ether but, instead, considerable quantities of the starting materials methanol and tert.-amylenes are present. In the case of ethers from even higher tertiary olefins, the position of the equilibrium towards the ethers is even more unfavourable. It is also established specialist knowledge that such ethers are cleaved again into the starting materials at even higher temperatures. The formation and the cleavage of the ethers, that is to say the establishment of the equilibrium at the particular temperatures, are influenced by acidic catalysts which quite frequently can be the same for both the formation and the cleavage, the temperature being in each case adjusted to that favourable for the formation or decomposition. The selective etherification of the tertiary olefins has also already been utilized for converting cracked gasolines to higher-octane motor fuels and motor fuel components by the formation of such ethers. In this case, the monounsaturated and polyunsaturated constituents of the cracked gasoline remain in such motor fuels or motor fuel components.
The highly unsaturated constituents of such mixtures are gum formers. In connection with motor fuels, gum is understood as a content of oligomeric or polymeric materials in the fuel, which gum is expressed as the evaporation residue in the analysis of the motor fuels. The gum content in motor fuel leads not only to carbonization and to deposits in the combustion chamber of the engine, but also covers the pores of an etherification catalyst during the manufacture of such motor fuels, and hence reduces the activity of the catalyst and shortens its service life and in addition causes a yellow coloration of the motor fuel being formed.
A process has thus already been disclosed (DE-OS (German Published Specification) 3,538,564) which allows, simultaneously with the etherification of tertiary C.sub.5 -C.sub.8 -olefins present in crude hydrocarbon mixtures with C.sub.1 -C.sub.5 -alkanols, the highly unsaturated and highly reactive fractions, which are present in these crude hydrocarbon mixtures and are held responsible for the gum formation, to be hydrogenated by means of an addition of small quantities of hydrogen. Such a simultaneous etherification and hydrogenation under mild conditions is carried out on styrene/divinylbenzene resins which contain free sulphonic acid groups and elements of subgroups VI, VII or VIII of the periodic table of the elements, in the metallic state.
In the case of such a simultaneous etherification and partial hydrogenation of the highly unsaturated gum formers, the monoolefins remain in the reaction mixture. These monoolefins can be hydrogenated to the corresponding paraffins only under more severe reaction conditions. Such a hydrogenation is highly desirable, since the monoolefins, as sources of free radicals, are held responsible, in addition to other substances, for the damage to the ozone layer in the upper atmosphere. A still persisting high olefin content in the motor fuels used worldwide on a large scale must therefore, according to present understanding, increasingly contribute to the undesired further damage to this ozone layer. There has therefore been no lack of attempts to convert the olefins in the motor fuels as far as possible into the corresponding paraffins by hydrogenation. Since, according to expert knowledge, a hydrogenation of the monoolefins was feasible only under more severe conditions, such as higher temperature, higher hydrogen availability and higher pressure, destruction of the ethers, which increase the octane number and are therefore desired, of the tertiary olefins had always to be expected. Such a destruction was all the more to be feared, since the involvement of the catalyst supports, frequently rendered acidic, of the hydrogenation catalysts to be employed had also to be taken into consideration. Effective hydrogenation processes for the monoolefins while preserving the alkyl tert.-alkyl ethers therefore had to proceed under special conditions. Thus, DE-OS (German Published Specification) 3,526,443 describes a hydrogenation process for olefinic hydrocarbons, which are present as a mixture with alkyl tert.-alkyl ethers, wherein very specific catalysts are used which possess a hydrogenation-active component on a catalyst support having a specific surface area of more than 50 m.sup.2 /g and a pore diameter of predominantly &lt; 1000 nm.
From the expert understanding and in the knowledge of the prior efforts for successfully carrying out the etherification on the one hand and, on the other hand, the extensive hydrogenation of the olefins while preserving the ethers already formed, it was not to be expected that, beyond the easily feasible partial hydrogenation of only the highly unsaturated compounds during the etherification, extensive hydrogenation of the monoolefins which are more difficult to hydrogenate could also be carried out, in which case it was necessary to meet the following requirements:
1. the monoolefins without a tertiary carbon atom, which are present, should be extensively hydrogenated without hydrogenation of the tertiary olefins to be etherified so that the maximum possible yield of alkyl tert.-alkyl ether is achieved; PA0 2. the more severe reaction conditions, which also include a higher hydrogenation temperature than that for the partial hydrogenation, should not lead to a cleavage of ethers, once formed, into the starting products alkanol and tertiary olefin; and PA0 3. the highly exothermic heat of reaction of the hydrogenation should not lead to overheating of the reaction mixture, which would reinforce the trends listed under 2., which had to be feared.